Too often beginning screenwriters attempt to push their story along with dialogue. "Hey, let's go to the racetrack?" "Sure, I wanna go." "How are we gonna get there?" "Maybe take the bus." "Okay, fine. Let's go!"
Just cut to the racetrack. We don't need all the chatter about whether characters want to go somewhere, how they're going to get there, et cetera.
One challenge I have for beginning screenwriters is to write your story as much as you can without using any dialogue at all.
The screenplay is a visual form. All it is a map for a medium that is going to end up telling your story in a visual way anyhow. So what can you say through visuals to begin with? How can the characters react to each other in ways that aren't verbal?
The best way is to have them react through action. I don't mean a bunch of gestures, which you don't really need to note in your script anyway ("he raised his hand", "he pointed his finger") and which the director and the actor are going to figure out later without the writer's help.
What I mean is action that is defined by what choices a character makes. Say, character X tells character Y she wants to break up with him. Instead of having him say something in response, have him DO something. Does he throw water in her face? Does he stand up and kiss another girl? Does he fall down at her feet and cry? Or do we just see him leave, get a gun and then shoot up the next scene? It's up to you.
If you are writing a comedy, please try to use as much physical humor in your comedy as you can. Joke telling is an art in itself, but it's not always funny in movies. Sure, there are lots of movies with really witty dialogue, but wouldn't you rather have the action of your scenes be what's funny -- what's happening, what your characters are doing -- than just falling back on two talking heads making jokes to one another? Unless that's your schitck, of course.
So -- lesson of the day. Action is humor. Action is drama. Action is key. The more you make your characters actually do something in your script, the more effectively you will move your story along, and the more appealing you will make your script to all those reading it. And wouldn't you like to differentiate your script from the pile of amateur screenplays in any reader's stack?